Mining with Backfill

This article deals with the usage of backfill in underground mine design. For the article on backfill see Backfill.

Mining with backfill is a requirement in the industry as it is an important aspect of many mining methods and is a solution to many rock stability issues. When a mine plans to use backfill there are a number of considerations to be made with regards to type of fill, delivery systems and from where the fill will be obtained. Furthermore, the application of fill will result in additional infrastructure above and underground which will ultimately lead to an overall increase in operation costs.

Sources of Backfill

There are various sources used by mines for backfill material. Mines normally make their choices based on what is locally available and most economical. Mine tailings are widely used in today’s industry, as they result in the lowest cost.[1] This is because tailings are available, and the use of tailings underground requires smaller tailings treatment and impoundment facilities on surface. Also, using tailings eliminates the need to transport materials from other locations. The use of waste rock is a popular option for rock fill methods, as it is readily available. However, when using the aforementioned materials, it is important that there are no sulphides or potentially harmful chemicals present within them. These issues can be avoided by separating materials on surface and being aware of the chemistry within each type of material. It is important to note that the use of waste materials for backfill can pose production constraints. If there is a shortage of backfill material, due to a low mill throughput, the mine production will be delayed while waiting for sufficient backfill amounts. Therefore, it is important to do detailed design and scheduling of production and backfilling activities, so that operational delays are avoided. This must be an ongoing review process, as some backfill sources, which were previously uneconomical, might become more competitive and profitable to exploit.[1]

When waste material is not readily available, the material must be found elsewhere. There are generally two options to acquire the materials; importing them from an outside source or quarrying near the mine site. Both of these methods incur additional costs. This is due to transportation costs and purchasing costs, when buying the material from another mill. It should also be noted that in Canada, there has been a lot of glaciation, which left behind glacial formations such as eskers that contain large amounts of fine particles. These can be exploited as a backfill material source.
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Paste Fill vs. Hydraulic Fill

Hydraulic fill has historically been preferred over paste fill. This is because it was easier to pour, making it the conventional fill type used by the mining industry. However, paste fill is seeing increased usage lately, as hydraulic and paste fills are equally used in the Ontario mining industry today. Due to new delivery systems technologies, paste fill capital costs have been decreased. The main distinguishing feature of paste fill is its necessity to be gravity fed to where it is needed.[2] It is not a fluid and it can’t be pumped.

An advantage of paste fill is that if there is ever a pipeline blockage, it doesn’t settle in the pipe. In contrast, hydraulic fill rapidly settles in the pipe, causing a blockage. Hydraulic fill also requires water handling. The water that is used underground must be pumped back to surface, which is an additional cost. Failures can also occur due to water because if it doesn’t consolidate and it fails, and fatalities may occur. Formerly, hydraulic fill was the easier method but paste fill is becoming more competitive. For the same cement content, paste fill has a higher strength. This reduces costs, as less cement is needed to fulfill the same support requirements.

Fill Delivery Systems

The first configuration deals with a direct pipeline from the fill station underground without any necessary infrastructure above ground. An advantage to this type of delivery is the velocity gained through the vertical distance traveled which often results in a horizontal delivery twice as long as the vertical drop. Due to the rapid flow however, measures must sometimes be taken to slow the fill down as to not damage the pipeline. Furthermore, if the ratio of vertical to horizontal distance is disproportionate, a pump may be necessary to push the fill to the required location. This can be seen in the above schematic, adding additional energy and maintenance costs as well.[4]

The second configuration is the most complex with regards to pipeline and will require the most pipe maintenance. Although the amount of possible pipe wear and failure is disadvantageous, this pipe layout allowed for a progressive amount of pressure to the pipes through its vertical-horizontal transportation. This arrangement is also beneficial as it is contained underground and leaves the possibility of circuit expansion as the mine operation grows.[4]

The third configuration comes with an advantage that everything is above ground where a pump is easily installed and maintained. A pump above ground as seen in the schematic also allows for the fill to be transported wherever necessary and then directly underground. However, this type of system is heavily reliable on the pump and with any stoppage in operation or problem due to weather effects; there would be no way to transport the fill underground. Furthermore, a large borehole is necessary to pump the fill underground and therefore requires a high pressure take-off point.[4]

Issues Delivering Fill Underground

Pipe Wear

Pipe wear occurs when part of the pipe wall is removed by the slurry through abrasion, erosion, or corrosion. When delivering the fill underground, an engineer must consider the effect that the slurry has on the pipes and the deterioration that will occur. Removal by abrasion occurs when a particle moves contiguously along the pipeline, essentially wearing it down until material from the pipe is removed. Erosive wear is the process by which a particle repeatedly smashes into the pipe in a specific area until pipe material eventually breaks away. Due to the nature of corrosion, where chemical reactions reduce the material on metal pipe walls, it is the leading cause for pipe wall wear and pipe failure as it can occur even when the pipe is idle. Furthermore, corrosion accounts for as much as 80% of the wear rate of pipelines used in backfill transportation.

Transporting Slurry Underground

The major causes that lead to pipe wear are a combination of slurry velocity, slurry density, pipe diameter, and pipe gradient. At low solids concentrations (<15% volume) the wear rate has been seen to increase linearly. However, it has also been found that at a high solids concentrations (>35% volume) the wear rate decreased.[4] This is largely due to the slurry’s relative density. As the density increases, the movement of its comprising particles within the slurry are slowed down, resulting in decreased wear rate.

Slurry velocity is the biggest factor to consider with regards to pipe wear rate and transporting fill underground. The wear rate of the pipes increases exponentially as the slurry velocity increases, according to the following relationship:

W = cVP

In this equation, W is the wear rate (m/s), V is the flow velocity (m/s), and c and p are determined experimentally as shown in the following table.

It can be shown that increasing the pipe diameter will decrease the pipe wear rate. This is due to the decrease in pipe pressure which leads to a decrease in shear stress. By reducing the shear stress along the pipe wall, the effects of abrasion from the slurry are diminished, which will lead to a reduction in corrosion and erosion as well.

Backfill Settling Times

Pouring Fill

When hydraulic fill is first poured into the stope, there is a minimum time that must be allowed for it to settle. This is to allow the water to decant and for consolidation of the cement to begin. Generally, a mine will wait one day or one shift for the fill to come in. The backfill will not be poured more than one metre per hour because if it was poured faster, it could liquefy and fail at the barricades. Once this is completed, filling may begin again. However, the actual settling or curing time of the material is dependent on what is going to be used for. If there is going to be work done on top of it, then it must have a certain bearing capacity and contain a greater cement content. The cement has to consolidate strong enough to be able to get binders coming in. If the cement is not reacting with anything, then there is no lengthy time period to wait. If it is simply being used to fill a stope, it will be usually be ready in 3-4 days.

Settling Time Issues

Where a mine is using a slurry fill, settling time is of great importance. The fill has to consolidate and the water must be forcefully squeezed out so that the fill can condense. As a rule of thumb, the water should be out of the pour within 24 hours. If the water is not entirely removed from the fill then there may be slime within the fill, preventing drainage. This will cause the fill to retain moisture and tends to form a gel-like material, which can be extremely dangerous. It is imperative that the fill is a uniform blend and one must ensure that the fines used in the blend are not overdone, as this can lead to a plug in the water pathways.

To avoid settling time issues, engineers often use flocculants or accelerants in the fill. Accelerants are used to increase or enhance the speed at which the cement sets. This is important because once the cements sets, the presence of fines can be neglected because the cement bond is in place. However if there is a fill with low cement content or no cement at all, then the water must be removed immediately to add flocculating agents. The agents cause big globs that have large pore spaces in between them in order for the water to decant freely. [2]

However, there are potential issues that can increase the settling time. Although engineers try to avoid these problems, the nature of materials and mixing can still cause constraints such as:

Physical Constraints

Presence of clay in water

Too much water in the mix

Restricted flow paths where the material is pinched together (leaving ponds of water in the material

Economical Evaluation

The cost of backfill is generally not a major cost to the entire mining process, depending on the type of backfill method being used. The costs can be expressed in either dollars per tonne of backfill or tonne of ore. The cost ranges from 1.6% and 16.4% of the total mining cost. This is very dependent on the backfill method being used, as there is a wide range of backfill applications. A slurry fill, for example, may require a capital cost of anywhere from $400,000 to $800,000, whereas a paste fill can range between $500,000 and $1,000,000 or greater.[4]

In order to determine the requirements of designing a backfill system, a conceptual model must be evaluated. This model should include the material available, the location of the mine, and the mining operation. In this evaluation, a copper orebody is located between 500m and 1,000m, and is mined with a 2% grade producing 1,880 tonnes of ore per day and 1,840 tonnes of tailings or 607,200 tonnes per year (assuming a 330 day operational year). Over the life of the mine, 600,000 tonnes of copper will be mined using strictly a longhole method and waste rock will be available at a rate of 185 tonnes/day (See Transverse longhole stoping). Also, tailings are acid generating and a tailings pond will need to be maintained 50 years after closure. Mill tailings and waste rock are the material that will be used, and the backfill strength requirement of 1 MPa will be assumed.[4]

In cut and fill operations, a high cement concentration is necessary to increase the curing times. This leads to bulkheads and cement consumption being the highest portion of the operating costs. It should be noted that the high cement cost is due to the requirement of 1 Mpa, and on average the cost of cement is about half the stated value.

General Fill Costs

Slurry fill has a large amount of cleanup costs which accounts for its high expense relative to other backfill methods. However, these costs are often allocated to mining costs instead of backfill costs. A paste backfill may initially have high capital costs due to the need for dewatering technology and concrete pumps, but tend to have lower operating costs This may, over time, allow for the initial capital costs necessary. A rock fill may cost less than a slurry or paste fill due to the reduced binder consumption and the elimination of expenses related to the transportation of water used in slurries.[6]

It should be noted that in Rock Fill Case 1, their high fill preparation costs are due to the particular operation’s need to quarry and crush surface rock in order to obtain the necessary waste rock.[4]Return to Top

Bulkheads

Bulkheads are a critical part of using backfill, as they are used as a retaining wall to hold the fill within the stope. There are generally two types of bulkheads; wooden and concrete. Although they are all designed to perform the same purpose, they are designed to be very site-specific, which leads to highly variable costs. These costs are also affected by type of backfill used and the mining method used.

Types of Bulkheads

Wooden bulkheads are the least expensive barricades, and are available to most locations in Canada due to the availability of timber. They are the easiest and fastest to install of the various designs. However, they are subject to possible deterioration, depending on the working environment. Wooden bulkheads are designed to hold up against pressures up to 207 kPa and have a higher deflection ratio of load compared to concrete and steel bulkheads. Concrete block bulkheads are the strongest type, withstanding up to 2,758 kPa, but come with much higher costs than wooden bulkheads.[3] Concrete bulkheads are often anchored for reinforced strength. They also require a 28 day curing period before any pressure can be applied to the bulkhead. This, in combination with their high costs, makes them generally less competative than wooden types.[3]

Bulkhead Costs

Bulkhead costs will vary greatly, which will be determined by the material or the type of backfill used. Bulkhead construction is also very labor intensive and can take weeks, depending on the barrier design. In a cut and fill operation, bulkheads can account for almost half of the operating costs, due to the high number of bulkheads necessary. For mining methods such as long hole, where backfill is not used as frequently, expenses can range anywhere from 2% to 7% of the operating costs.[3]Return to Top

This article was designed and created by Group 4 (Eric Archibald, David Stewart, Victor DeJulio, Brendan Bureau and Wei Duan) as part of the MINE 448 Underground Mine Design Course instructed by Professor Stephen McKinnon at Queen's University.